Method &amp; means of forming threaded ties and rods

ABSTRACT

Method and means of manufacturing ties, fasteners and rods ( 15 ) having a plurality of longitudinal threads by forcing a coil of roll-profiled feed-wire made of steel ( 11 ) through a twisting-die made of plastic ( 1 ). Also described is: a twisting-die made of plastic ( 1 ) that is suitable for twisting profiled feed-wire ( 11 ) made of steel; a method of forming a plastic twisting die ( 1 ) using an driven tap ( 31 ) in the form of a twisted rod ( 15 ); and a helically-shaped member ( 15 ) having lead measurements (X) along the length of the helical thread that vary less than pitch measurements (Y) along the lengths of the helical threads.

FIELD OF THE INVENTION

The present invention is directed to the subject matter of manufacturingties and rods having a plurality of longitudinal threads by forcing acoil of roll-profiled feed-wire made of steel through a twisting-diemade of plastic.

BACKGROUND OF THE INVENTION

Twisting-dies have typically been designed as multiple-segment machinedmetal components which are assembled to form a hollow die having aplurality of internal helical troughs. The machining, assembly and useof such multi-segment twisting-dies is problematic.

Production of dies having a plurality of internal helical grooveswherein each groove gradually increases in angle from one end of the dieto the other is intricate, slow and expensive.

Another problem is that flange-profiled feed-wire is typically woundonto a reel and therefore is coiled rather than straight. The wire hascurvatures in two dimensions; the first curvature is caused by the wirerunning circularly in a coil and the second, a less severe curvature, iscaused by the wire running along the breadth or height of the coil, fromone end of the reel to the other. These curvatures prevent the leadingend of a coil of wire being advanced from the reel directly into andthrough a die.

A further problem is that flanged steel wire is predisposed to bucklingbetween the force that pushes the wire and the twisting interface withinthe steel die. This is because a substantial force is required to pushflange-profiled steel wire in an axial direction through a twisting diein order to deform it helically. Extreme pressure effectively squeezesout any lubrication at the twisting interface and forces it out of thedie at the segment joints or seams, thus intensifying the effects offriction as steel slides on steel. In addition to the direct metal tometal contact causing significant surface damage on one or more of thesliding surfaces, the effect of friction increases the force required topush the wire through the die to beyond critical limits. This causes thewire to buckle such that it becomes jammed within the die or supportguide.

Typically, the dry static friction coefficient for steel on steel is0.8μ. Once sliding is initiated friction is reduced; the coefficient ofkinetic friction for steel on steel is approximately 0.4μ. As highervelocity is introduced into the twisting process in order to twist thesteel feed-wire at commercially viable rates, the pressure, heat and thecoefficient of friction at the twisting interface will increase.

An example of manufacturing helical connecting devices is disclosed inU.S. Pat. No. 7,269,987 (Ollis). The method includes pushing the leadingend of a wire, rod or hollow extrusions straight into and through ahelical deformation arrangement comprising a die. The die has a straightentry guide-portion leading to internal helical passageways having aprogressive acceleration of helical compound angles. The helicalpassageways require a set of at least nine helical broaching tools toform more than forty deflection nodes needed to steadily reduce thepitch distance between the helical passageways and a small number ofstraightener tools. A die formed by broaching a progressive accelerationof helical angles can only be formed as a multi-segment die.

Other examples of using segmental metal twisting-dies to twist steelrods are disclosed in U.S. Pat. No. 497,827 (Sellers) and U.S. Pat. No.1,549,140 (McCurdy), albeit the subject matter relates to making drillbits and augers.

U.S. Pat. No. 497,827 (Sellers) discloses a method and means of making adrill bit using a multi-part support-guide and a multi-part twisting dieinto which individual lengths of straight flanged rod are pushed bydrive-rollers to give the rod a loose helical twist along a portion ofits length, for example, a drill bit with a full 360-degree twist over adistance of 9 inches (228 mm). The sixteen-part twisting die isdismantled in order to withdraw the part-twisted drill bit from the die.

U.S. Pat. No. 1,549,140 (McCurdy) discloses a method and means of makinga drill bit or auger using a two-part twisting-die having straight guidegrooves leading to milled helical troughs with a progressivelydecreasing lead angle to gradually reduce the helical pitch distance.The two parts are welded together to form a composite die through whichindividual straight lengths of a flat bar are driven with a ram toprogressively form a helix-shaped drill bit.

Accordingly, the invention seeks to provide a more effective method andmeans of manufacturing loosely threaded helically-shaped members atcommercially viable rates. This can be achieved by twistingflange-profiled feed-wire through a low-friction twisting die. Such adie can be quick, simple and inexpensive to produce and can eliminateleakage, minimise surface damage and relieve the obstacle of buckling.

SUMMARY OF THE INVENTION

The invention finds significant utility in manufacturinghelically-shaped members, for example in the form of at least one ofthreaded fasteners, threaded ties and threaded rods by forcing profiledmetal feed-wire, which is typically made from steel, having a pluralityof longitudinal flanges through a new type of twisting-die. The newtwisting-die made of plastic.

Plastic can be formulated such that the coefficient of friction declineswith increasing loads. It can also be formulated to offer excellentabrasion and wear resistance in high-load applications. Plastic twistingdies facilitate faster through-put rates than can be achieved with diesmade of steel, they use less force to push flange-profiled feed-wirethrough the die and they alleviate surface damage to the feed-wire as itslides on the plastic and advances through the die.

Another advantage of using plastic twisting dies to twist steelfeed-wire is that the helical grooves within the die do not need toprogressively increase in angle from one end of the die to the other.Without being limited by any particular theory, this is thought to be byvirtue of the plastic die's elastically compliant constitution. Plastichas a lower modulus of elasticity than steel and is sufficientlycompliant to accommodate curvatures on the feed-wire and appliedstresses at the twisting interface. The lower modulus of elasticity,combined with the lower coefficient of friction between the selectedplastic material and metal, such as steel, permits the twisting offlange-profiled feed-wire within helical grooves where each groove has asubstantially uniform helix angle along the entire length of the groove,and hence a substantially uniform lead angle along the entire length ofthe groove. The plastic die eliminates the need to machine complexhelical grooves which gradually tighten as they extend through thelength of the die, making the production of the dies quick, simple andinexpensive. The die can be produced from a single piece of material toeliminate the prospect of joint leakage. In use, the plastic die is aviable and cost-beneficial alternative to using a steel die to twistprofiled steel feed-wire.

A further advantage of using a plastic die is its capacity to cope withrepetitive stop and start twisting, such as may be required when forcinga continuous coil of profiled feed-wire through the die. Alternating‘stop-go’ processes result in recurring changes between static andkinetic coefficients of friction at the twisting interface as thefeed-wire advances into and through the die. A low-friction twisting diepermits the flange-profiled feed-wire to travel smoothly through theplastic die with reduced force, thereby eliminating buckling or jammingof the feed-wire.

The inventors believe that the current invention represents significantimprovements in production capability, cost-effectiveness, speed andsimplicity.

The essential elements of the inventions are defined by the independentclaims and the advantageous embodiments are distinguished in thedependent claims.

In view of the forgoing, according to one aspect there is provided amethod for manufacturing helically-shaped members, for example in theform of at least one of: threaded fasteners, thread ties and threadedrods.

The method can include providing profiled feed-wire comprising metal,the feed-wire having a central core and a plurality of longitudinalflanges extending outwards from the core and running parallel to thecore.

The method can include providing a die body made of plastic, the diebody having an axial cavity formed through the die body and a pluralityof internal helical grooves formed in the die body. Each helical groovecan extend substantially the whole length of the axial cavity at asubstantially uniform helix angle.

The method can include engaging at least some of the flanges withinrespective internal helical grooves, and forcing the profiled feed-wirein an axial direction through the die body, whereby a plastic surfacewithin at least one of the helical grooves deflects its respectiveflange thereby forcing the profiled feed-wire to helically deform as itadvances through the die body.

The method can include extruding a helically-shaped member, for examplein the form of a helically-shaped wire, from the die body. Thus theplastic die twists the feed-wire to produce the helically-shaped wire.

The die body can comprise a single piece of material. Alternatively, thedie body can comprise a plurality of pieces of material joined together.

The plastic can comprise synthetic polymer. The die body can be madefrom a single synthetic polymer or a plurality of synthetic polymers.

The plastic can comprise a self-lubricating plastic. The die body caninclude an integral lubricant dispersed throughout its matrix.

The plastic can include a polyamide, and preferably can include nylon.The die body can be made of nylon.

The profiled feed-wire can be in the form of a continuous coil woundonto a reel.

In some embodiments each flange extends radially outwards from the core.In some embodiments, at least some of the flanges extend outwards fromthe core but in a manner that is offset from a centre of the core, forexample, the flanges can be arranged sustainably parallel to a radius ofthe core.

The profiled feed-wire can include steel, and preferably stainless steeland/or high-tensile steel. The profiled feed-wire can be made of steel.

The profiled feed-wire can include stainless-steel having an austeniticcrystalline structure. The profiled feed-wire can be made ofstainless-steel having an austenitic crystalline structure.

The modulus of elasticity of the feed-wire may be greater than or equalto 180 GPa. Steels typically have a modulus of elasticity around 180 GPato 200 GPa.

The modulus of elasticity of the feed-wire can be greater than themodulus of elasticity of the die body. The modulus of elasticity of thefeed-wire can be at least twenty percent greater than the modulus ofelasticity of the die body, and preferably at least one hundred percentgreater than the modulus of elasticity of the die body. In someembodiments, the modulus of elasticity of the feed-wire can be at leastten times greater than the modulus of elasticity of the die body.

The axial cavity can have a uniform transverse cross-section along itslength. For example, the transverse cross-section can be substantiallycircular.

The axial cavity can be substantially cylindrical. This shape is wellsuited to the feed-wire core.

The grooves can extend outwards from the axial cavity. In someembodiments, in a transverse cross-section of the die body, the groovesextend radially outwards from the core.

The method can include helically deforming a leading end of the profiledfeed-wire before inserting it into the twisting-die. This can reducewear on the die body. For example, this step can be performed manually,and may include the use of a separate die and/or other tools.

The die body can have a cylindrical shape. An outer diameter of the diebody can be at least three times a circumscribed diameter of theprofiled feed-wire. This helps to maintain the structural integrity ofthe die body during an extrusion process.

The plurality of longitudinal flanges can include a plurality of majorlongitudinal flanges and a plurality of minor longitudinal flanges. Eachmajor flange can have a greater depth than the depth of each minorflange. The flange depth is measured in a generally radial directionfrom a root of the flange to a tip of the flange. Each minor flange canbe located between two major flanges. Each major flange can be insertedinto a respective helical groove in the die body. Each minor flange canbe inserted into the axial cavity. Thus in some embodiments the die bodycan have fewer helical grooves than the feed-wire has flanges. Thenumber of grooves typically matches the number of major flanges.

The depth of the minor flanges and the size (diameter) of the axialcavity can be dimensioned to provide a clearance between an axial cavitywall and peripheral portions of the minor flanges.

An inscribed diameter of the axial cavity within the die body can be atleast 30% greater than an inscribed diameter of the central core of theprofiled feed-wire. This helps to accommodate the physical changes thatoccur to the feed-wire during the twisting process.

The profiled feed-wire can be curved in form.

The method can include winding the die body on to the leading end of acoil of feed-wire and locking the pre-engaged die body against rotationwith a die holder. This helps to prevent the die body from wearing.

The method can include driving the profiled feed-wire through the diebody by powered drive rollers.

The method can include temporarily halting rotation of the powered driverollers during an extrusion process, and subsequently restarting thepowered drive rollers.

The profiled feed-wire can be twisted through at least one full rotationwithin the confines of the die body, and preferably can be twistedthrough more than one full rotation within the confines of the die body.For example, the feed-wire can be twisted through greater than or equalto 1.25 rotations within the confines of the die body. Each helicalgroove applies turning moments to its respective flange, which causesthe axial core to twist about its longitudinal axis, thereby helicallydeforming the feed-wire.

In some embodiments, the profiled feed-wire is twisted through less thanor equal to 10 full rotations within the confines of the die body.

The coefficient of friction for steel sliding on the plastic used tomake the die body can be less than the coefficient of friction of steelsliding on steel. This provides a lower friction arrangement than makinga die out of steel, and thus requires less force to push the feed-wirethrough the plastic die body.

The plastic used to make the die body can have a static coefficient offriction on steel of less than or equal to approximately 0.3μ. Theplastic used to make the die body can have a kinetic coefficient offriction on steel of less than or equal to approximately 0.2μ.

The method can include driving the profiled feed-wire through the diebody at a throughput rate in the range 100 mm to 500 mm per second.

The helically-shaped wire that is extruded from the die body can have agreater mass per unit length than the profiled feed-wire that can be fedinto the die body. The transverse cross-sectional area of the profiledfeed-wire can be at least 10% less than the cross-sectional area of thestock-wire, and can be at least 12% less than the cross-sectional areaof the stock-wire.

The method can include cutting the helically-shaped wire to form, forexample, at least one of: threaded fasteners, threaded ties and threadedrods. The helically-shaped members each have an axial core and aplurality of longitudinal helical threads.

The method can include cutting the helically-shaped wire after thepowered drive rollers have been halted. The method can include acontroller synchronising operation of a cutting device with the powereddrive rollers. The controller can be arranged to halt rotation of thedrive rollers and subsequently actuate the cutting device to cut theextruded helically-shaped wire.

The method can include a preparatory step of providing stock-wire. Thestock-wire can have a substantially circular transverse cross-section.The method can include progressively squeezing the stock-wire betweenshaping rollers on a plurality of occasions such that the transversecross-sectional shape of the stock-wire is gradually plasticallydeformed into the transverse cross-section of the profiled feed-wire,without cutting or shearing the surface of the wire.

The tensile strength of the profiled feed-wire can be in the range 1.7to 2.3 times the tensile strength of the stock-wire.

The profiled feed-wire can have an ultimate tensile strength in therange 850 MPa to 1380 MPa.

At least one of the flanges can be work-hardened. The or eachwork-hardened flange extends outwards, and in some embodiments radiallyoutwards, from a softer core.

The hardness of at least one flange can be in the range 25 to 45 on theRockwell C scale.

At least one, and preferably more than one, flange can be elongate intransverse cross-section.

At least one, and preferably more than one, flange can have a greaterhardness than the die body.

At least one, and preferably more than one, flange can taper along itsdepth from a wider portion towards a root portion to a narrower width ata tip portion.

The transverse cross-section of the flanges can vary in depth, width orshape.

The plastic die body can have characteristics according to the secondaspect of the invention.

The die body can be formed in a preceding step according a third aspectof the invention.

The helically-shaped members can have characteristics according to thefourth aspect of the invention.

According to a second aspect, there is provided a twisting-die that issuitable for twisting profiled feed-wire made of steel. The twisting-dieincludes: a die body made of plastic. The die body can include asubstantially axial cavity formed through the die body. The die body caninclude a plurality of internal helical grooves formed within the diebody. Each helical groove can extend along substantially the wholelength of the axial cavity at a substantially uniform helix angle.

The die body can comprise a single piece of material. Alternatively, thedie body can comprise a plurality of pieces of material.

The plastic can comprise synthetic polymer. The die body can be madefrom a single synthetic polymer or a plurality of synthetic polymers.

The plastic can comprise a self-lubricating plastic. The die body caninclude an integral lubricant dispersed throughout its matrix.

The plastic can include a polyamide, and preferably can include nylon.The die body can be made of nylon.

The plastic used to form the die body can have a static coefficient offriction on steel of less than or equal to approximately 0.3μ.

The plastic used to form the die body can have a kinetic coefficient offriction on steel of less than or equal to approximately 0.2μ.

The die body can include reinforcement. The die body can includemicrosphere reinforcement. The die body can include fibre reinforcement.The die body an include at least one of glass fibres, carbon fibres,basalt fibres and aramid fibres.

The hardness of the die body can be in the range 70 to 95 on the Shore Dscale.

The die body can be made from plastic having a modulus of elasticitythat can be less than or equal to 18 GPa, preferably less than or equalto 15 GPa, more preferably less than or equal to 10 GPa, and morepreferably still less than or equal to 6 GPa.

Alternatively, the die body can be made from reinforced plastic having amodulus of elasticity that can be less than or equal to 150 GPa,preferably less than or equal to 100 GPa, more preferably less than orequal to 50 GPa.

At least one, and preferably each one, of the internal grooves can turnthrough at least one full rotation within the confines of the die body,and preferably can turn through more than one full rotation within thedie body.

At least one, and preferably each one, of the internal grooves withinthe die body can turn through at least one full rotation over an axialdistance in the range 20 mm to 75 mm. In some embodiments, the groovesturn through a maximum of 4 revolutions over an axial distance in therange 20 mm to 75 mm.

Each helical groove can extend outwardly from the axial cavity. Eachhelical groove can be contiguous with the cavity. Each helical groovecan have a root portion adjacent the axial cavity. Each helical groovecan be open at the root portion. Each helical groove can have a tipportion distal from the axial cavity. Each helical groove can be closedat the tip portion.

In some embodiments, when the die body is viewed in transversecross-section, at least one, and preferably each one, of the internalgrooves extends radially outwards from the axial cavity. In otherembodiments, when the die body is viewed in transverse cross-section, atleast one, and preferably each one, of the internal grooves extendsparallel to a radius of the axial cavity.

In some embodiments, when the die body is viewed in transversecross-section, at least one, and preferably each one, of the internalgrooves can taper radially from the root portion adjacent the cavity tothe tip portion distal from the axial cavity.

In some embodiments, when the die body is viewed in transversecross-section, the radial depth of at least one, and preferably eachone, of the internal grooves can be less than the radius of the axialcavity. In some embodiments, when the die body is viewed in transversecross-section, the radial depth of at least one, and preferably eachone, of the internal grooves can be less than or equal to 4 mm.

In some embodiments, when the die body is viewed in transversecross-section, the cross-sectional area of at least one, and preferablyeach one, of the internal grooves can be approximately 7.5% or less ofthe inscribed cross-sectional area of the cylindrical cavity.

In some embodiments, when the die body is viewed in transversecross-section, the cross-sectional area of at least one, and preferablyeach one, of the internal grooves can be less than 8 square millimetres.

When the die body is viewed in longitudinal cross-section, the grooveopenings in the wall of the cavity can be spaced apart at an axialdistance in the range 10 mm to 35 mm.

When the die body is viewed in longitudinal cross-section, the length ofeach land located between one internal groove and the next internalgroove can be greater than or equal to 12 times the width of the grooveopening.

The die body can be cylindrical, and an outer diameter of the die bodycan be greater than three times a circumscribed diameter of the helicalgrooves.

The axial cavity can have a uniform transverse cross-section along itslength. The axial cavity can have a circular transverse cross-section.The axial cavity can be substantially cylindrical. An inscribed diameterof the substantially cylindrical cavity can be in the range 2 mm to 9mm.

The die body can have a countersink at one end of the axial cavity. Thedie body can have a countersink at each end of the axial cavity.

The plastic twisting die can be formed according to the third aspect ofthe invention.

According to a third aspect, there is provided a method of producing aplastic twisting-die that is suitable for twisting profiled feed-wiremade of steel. The method can include providing a body made fromplastic, the body having an axial cavity. The method can includeproviding a tap having an axial core and a plurality of helical threadsextending along the axial core. The method can include aligning theaxial core of the tap with the axial cavity. The method can include theplurality of helical threads cutting a plurality of internal helicalgrooves into the body. Each internal helical groove can extendsubstantially the whole length of the cavity. Each internal helicalgroove can have a substantially uniform helix angle. The method caninclude removing the tap from the body.

Each helical thread can have a lead angle in the range 50 degrees to 70degrees.

Each helical thread can have, in transverse cross-section, an angledefined by the apex at the tip of a thread in the range 5 degrees to 30degrees.

The plastic can comprise synthetic polymer. The die body can be madefrom a single synthetic polymer or a plurality of synthetic polymers.

The plastic can comprise a self-lubricating plastic. The die body caninclude an integral lubricant dispersed throughout its matrix.

The plastic can include a polyamide, and preferably can include nylon.The die body can be made of nylon.

The tap can turn through at least one full rotation within the confinesof the die body, thereby cutting internal helical grooves that turnthrough at least one full rotation within the die body.

The tap can turn through more than one full rotation within the confinesof the die body, thereby cutting internal helical grooves that turnthrough more than one full rotation within the die body.

The leading end of the tap can be shaped to help centre the tap to alongitudinal axis of the axial cavity, at one end of the die. Theleading end of the tap can narrow in width, for example, tapered/roundedinto a point or chisel tip.

The method can include forcing the tap in an axial direction into theplastic body. For example, a portion of helically-shaped wire extrudedfrom a plastic die according to the invention can be adapted for use asa thread-cutting tap for forming the plurality of helical grooves in afurther plastic body, to form a new plastic die, which itself can beused to produce further helically-shaped wire.

Typically, an external rotational force is not applied to the tap whenforcing the tap into the body. For example, the tap can be percussivelydriven or pressed into the body. The body is typically restrained fromrotating when the tap is percussively driven or pressed into the diebody.

The method can include forcing the body in an axial direction on to thetap. In some methods, the body can be percussively driven or pressed onto the tap. The tap is typically restrained from rotating. Typically, anexternal rotational force is not applied to the body when forcing thebody on to the tap.

The method can include forming a countersink in at least one end of theaxial cavity.

The tap can be formed in a preceding step according to the first aspect.That is, the tap can be formed from helically-shaped wire extruded fromthe plastic die in the first aspect.

According to a fourth aspect there is provided a helical member havingan axial core and a plurality of helical threads extending along thelength of the axial core. The helical threads can be arranged such thatany variation in lead measurements along the length of at least one ofthe plurality of helical threads, can be less than a variation in pitchmeasurements along the lengths of the helical threads. The helicalthreads can extend along substantially the whole length of the axialcore. Each helical thread can have a substantially uniform helix angle.

The variation of the lead is calculated by subtracting the minimum leadmeasurement from the maximum lead measurement along the length of thehelical thread measured. The variation of the pitch is calculated bysubtracting the minimum pitch measurement from the maximum pitchmeasurement along the lengths of the threads.

The difference between the variation in pitch and lead can provide auseful mechanical property in some applications. For instance, it canassist with material such as mortar providing a mechanical interlockwith the plurality of helical threads.

The variation in the lead measurements of at least one of the helicalthreads can be less than half the variation in pitch measurements.

The helical member can have an ultimate tensile strength in the range850 MPa to 1380 MPa

The hardness of at least one helical thread can be in the range 25 to 45on the Rockwell C scale.

The helical member can be in the form of a fastener, tie or rod. In somearrangements, the helical member can be arranged to be percussivelydriven into a substrate. For example, the leading end of the helicalmember can be formed into a drivable tip, that can be driven into wood,mortar, masonry or insulation.

The helical member can include a plurality of major helical threads. Thehelical member can include a plurality of minor helical threads. Eachmajor thread can have a greater depth than each minor thread. The threaddepth is measured in a generally radial direction. Each minor thread canbe located between two major threads. The helical member can have a tipat each end. For example, the helical member can have a driving tip atone end and a tapered tip at another end. The tapered tip helpspercussive driving forces to be concentrated on the axial core of themember, thereby assisting to drive the helical member into a substrate.

The helical member can include a shank portion at one end. The shankportion can be formed by removing a portion of its major helical threadsat one end of the member. The shank portion can have substantiallyparallel sides along at least part of the length of the shank portion.The shank portion may have a maximum diameter that is more than 50% ofthe maximum diameter of the helical member. The shank portion may have amaximum length that is less than or equal to the length of a pitchdistance.

The shank portion can include a tapered portion at one end such thatpercussive driving forces can be concentrated on the core of the member.

The helical member can include a head. For example, the helical membermay have a head that is attached to or formed at one end of the member.

The axial core can have a transverse cross-section that is less than orequal to two-fifths of a transverse circumscribed cross-sectional areaof the helical threads.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is an end view of a plastic die according to the invention;

FIG. 2 is a longitudinal sectional view of the die of FIG. 1, showinginternal helical grooves and the land between the grooves;

FIGS. 3A to 3D are transverse cross-sectional views of various profiledfeed-wires that are suitable for twisting into helically-formed membersby means of respective plastic dies arranged in accordance with theinvention;

FIG. 4 shows a profiled feed-wire advancing through the plastic die ofFIG. 1 to produce a helically-shaped wire;

FIG. 5 is a schematic view of a production system including the plasticdie according to FIG. 1, that is arranged to produce helically-shapedwire from a profiled feed-wire;

FIG. 6 is a schematic diagram illustrating the difference between thelead and pitch for a fastener, tie or rod having a plurality of helicalthreads, and illustrates lead and helix angles; and

FIG. 7 is a schematic view illustrating the cutting, by means of a tap,of internal helical formations in the plastic die of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION The Plastic Twisting Die

FIG. 1 is a diagrammatic end view of a plastic die 1 according to theinvention. The die 1 is arranged to twist metallic profiled feed-wire(see FIGS. 3A to 3D) as it passes through the die 1 to form a helicalwire having a plurality of helical threads.

The die 1 is typically made from a single piece of plastic, for example,a cylindrical or cuboid block of plastic, which is referred to as thedie body. The block can be moulded, or may be cut from stock material.The die 1 is has a longitudinal axis, and is preferably elongate. Thedie 1 includes an axial cavity 2, such as a substantially cylindricalaxial cavity 2. The cavity 2 extends in a longitudinal direction of thedie 1 for the full length of the die, and is typically formed along acentral longitudinal axis. The cavity 2 is open at each end. The cavity2 typically has an inscribed diameter in the range 2 mm to 9 mm. The die1 includes a plurality of helical grooves 3. Each groove 3 extendsradially outwards from the axial cavity 2. Each groove 3 extends in ahelical fashion along the full length of the axial cavity 2. Typically,each helical groove 3 turns through more than one full rotation aboutthe axial cavity 2. The helical grooves 3 are spaced apart from oneanother about the circumference of the axial cavity 2. Each groove 3 hasa root portion 3 a adjacent the axial cavity 2. The root portion 3 a ofeach helical groove 3 is open to, and contiguous with, the axial cavity2. Each groove 3 has a tip portion 3 b distal from the axial cavity.Each tip portion 3 b is closed.

In the example shown in FIG. 1, the die 1 includes two helical grooves3. The helical grooves 3 are arranged diametrically opposite to oneanother about the axial cavity 2. Optionally, each groove 3 can betapered in cross-section, such that the groove 3 is wider at the rootportion 3 a and is narrower at the tip portion 3 b.

The plastic used to form the die preferably has, under normal load, acoefficient of static friction on steel of less than approximately 0.3μ.The plastic used to form the die preferably has, under normal load, acoefficient of kinetic friction for steel on plastic of less thanapproximately 0.2μ.

The die 1 can be made from a self-lubricating plastic, or at leastoperative portions of the die 1 can be made from a self-lubricatingplastic. For example, at least operative parts of the die 1, andtypically the whole die 1, can be made from a polyamide, such as nylon.Beneficially, the use of a self-lubricating plastic may considerablyincrease pressure-velocity capabilities and may improve wear resistanceof up to ten times when compared to plastics that are notself-lubricating. Improved wear characteristics ensure excellentretention of physical properties.

The modulus of elasticity of the plastic die material is typically lessthan or equal to 18 GPa, preferably less than or equal to 10 GPa, andmore preferably is less than or equal to 6 GPa.

Optionally, the plastic die 1 may include reinforcement. For example,the die 1 can include fibre or microsphere reinforcement, such glass,carbon, basalt or aramid reinforcement. In this case, the modulus ofelasticity of the die body can be substantially higher. For example,carbon fibre reinforced plastic can have a modulus of elasticity ofaround 150 GPa. Glass fibre reinforced plastic can have a modulus ofelasticity of around 20 GPa.

FIG. 2 represents the plastic twisting die of FIG. 1 as it would appearif cut lengthways to show its internal structure. FIG. 2 shows plasticlands 4 which form the internal wall of the cylindrical cavity areintersected by the helical grooves 3, each groove having a substantiallyuniform helix angle along substantially the full length of the die 1.

The dashed lines 5 represent the depth of the helical grooves 3 at theirouter periphery or tips 3 b. Typically, the helical grooves 3 extend upto a maximum depth of approximately 4 mm into the plastic die body 1.

The plastic die 1, which may also serve as a straightening die, needs tobe balanced in bulk, elastic compliance and strength by which to absorbloads and resist failure as pressure is exerted at the twistinginterface. To provide high load-bearing qualities and good dimensionalstability, the plastic used to form the die 1 preferably has a hardnessin the range 70 to 95 on the Shore D scale. The dimensions of the diebody 1 are sufficient to prevent failure of the die 1, for example toprevent it from cracking. Preferably die body 1 is cylindrical and hasan outer diameter that is greater than three times the circumscribeddiameter of the grooves 3. The width 18 of each one of the openings in anewly manufactured die is typically less than or equal to 3 mm, andpreferably less than or equal to 2 mm. The width 18 of the opening ismeasured in a direction that is perpendicular to the helix angle (seeFIG. 2). The width 18 of the opening can increase with use due to wear.The distance between adjacent groove openings at the root portion 3 b ina longitudinal axis direction, and hence the axial length of the land 4between adjacent groove openings, is typically in the range 10 mm to 35mm. In some arrangements, the axial length of the land 4 betweenadjacent groove openings at the root portion 3 b may be greater than orequal to 12 times the width of the groove opening at the root portion 3b. The axial length of land 4 between adjacent groove openings at theroot portions 3 b can vary along the length of the die in someembodiments, for example due to variations in pitch along the length ofa tap, which forms the helical grooves in the die body. In otherembodiments, the axial length of land 4 is substantially uniform alongthe length of the die.

The Feed-Wire

FIGS. 3A to 3D are cross-sectional views of examples of feed-wires 11that can be twisted into helical forms when forced through respectiveplastic dies arranged in accordance with the invention. Typically, thefeed-wires 11 include an axial core 12 and at least two longitudinalflanges 13. Each longitudinal flange 13 extends outwards from the axialcore 12, and typically extends outwards in a substantially radialdirection from the axial core 12.

However, in some embodiments the flanges 13 extend outwards from thecore 12 substantially parallel to a radius. The feed-wires 11 aretypically flange-profiled feed-wires 11, and preferably profiledfeed-wires 11 created by a rolling process acting on a stock-wire (notshown). Preferably, the feed-wires 11 are made from metal, andpreferably a metal having a high tensile strength. For example, thefeed-wires 11 can be made from steel, and preferably stainless steel. Aparticularly preferred material is a stainless steel having anaustenitic crystalline structure.

FIG. 3A shows the cross-sectional contour of a feed-wire 11 which can betwisted into a helically-shaped member, in the form of ahelically-shaped wire 15, when forced in an axial direction through thedie 1 of FIG. 1. The feed-wire 11 includes a central axial core 12. Thefeed-wire 11 includes two major flanges 13 extending radially outwardsfrom the core 12. The two major flanges 13 are arranged diametricallyopposite to one another about the core 12. Each major flange 13 extendsalong the full length of the core 12. The feed-wire 11 includes twominor flanges 14 extending radially outwards from the core 12. Eachminor flange 14 extends along the full length of the core 12. The twominor flanges 14 are arranged diametrically opposite to one anotherabout the core 12. The minor flanges 14 are located substantiallyequidistantly between the major flanges 13. The minor flanges 14 arearranged substantially perpendicularly to the major flanges 13. Thedepth 17 of each major flange 13 is greater than the depth 17 of eachminor flange 14. Depth 17 is measured in a radial direction, from a rootof the flange to a tip of the flange (while FIG. 3A illustrates flangedepth for major flanges 13, it will be appreciated that the measurementis also applicable to minor flanges 14). The circumferential width ofeach minor flange 14 is greater than the circumferential width of eachmajor flange 13. Thus the major flanges 13 are relatively slender intransverse cross-section since they have a greater depth 17 and narrowerwidth, and the minor flanges 14 are relatively squat in transversecross-section since they have a greater width and lesser depth 17.

Each major flange 13 is tapered in transverse cross-section, such thatthe flange 13 is wider at the root portion and is narrower at the tipportion.

It can be seen by comparing the transverse cross-section of thefeed-wire 11 to the die of FIG. 1 that the major flanges 13 are wellsuited to fitting within the helical grooves 3 in the die, whilst thecore 12 and the minor flanges 14 are well suited to fitting within thecylindrical axial cavity 2, which is represented by the dashed circularline in FIG. 3A.

FIG. 3B shows a cross-section of a feed-wire 11 having a central axialcore 12, two major pointed wedge-shaped flanges 13 extending radiallyoutwards from the axial core 12 and two pointed wedge-shaped minorflanges 14 extending radially outwards from the axial core 12. Eachmajor flange 13 extends along the full length of the core 12. Each minorflange 14 extends along the full length of the core 12. The two majorflanges 13 are arranged diametrically opposite to one another about thecore 12. The two minor flanges 14 are arranged diametrically opposite toone another about the core 12. The minor flanges 14 are locatedsubstantially equidistantly between the major flanges 13. The majorflanges 13 are arranged substantially perpendicularly to minor flanges14. The minor flanges 14 have a different shape from the major flanges13.

When the feed-wire 11 of FIG. 3B is used with a plastic die, thetransverse cross-sectional shape of each groove 3 is matched to theshape of the major flanges 13 when manufacturing the die 1. The majorflanges 13 will then be suited to fitting within the grooves 3 in thedie, whilst the core 12 and the minor flanges 14 are well suited tofitting within the cylindrical axial cavity 2, which is represented bythe dashed circular line in FIG. 3B.

FIG. 3C shows a cross-section of a feed-wire 11 having a central axialcore 12 and three major flanges 13 extending radially outwards from theaxial core 12. Each major flange 13 has a rounded tip, and substantiallythe same size and shape. Each major flange 13 has a radiused portion atits root and tapers to the tip. In this example, though not limited tofeed-wires having three major flanges, the major flanges 13 are unevenlydistributed about the core 12, such that the transverse cross-sectionalprofile is selectively rotationally asymmetric.

When the feed-wire 11 of FIG. 3C is used with a plastic die, the dieincludes 3 helical grooves 3 distributed about the axial cavity 2. Thetransverse cross-sectional shape of each groove 3 is matched to theshape of the major flanges 13 when manufacturing the die 1. The majorflanges 13 will then be suited to fitting within the grooves 3 in thedie, whilst the core 12 is well suited to fitting within the cylindricalaxial cavity 2, which is represented by the dashed circular line in FIG.3C.

When a rotationally asymmetric feed-wire is twisted, it can form ahelically shaped member having peak to peak pitch measurements that varyor alternate along the length of the wire. Notwithstanding thenon-uniform pitch measurements, each thread of the helically shapedmember can have a substantially uniform lead measurement alongsubstantially the full length of the wire.

FIG. 3D shows a feed-wire 11 having a central axial core 12, two majorflanges 13 extending outwards from the core 12, and two minor flanges 14extending outwards from the core 12. Each major flange 13 extends alongthe full length of the core 12. Each minor flange 14 extends along thefull length of the core 12. The minor flanges 14 are located between themajor flanges 13.

The transverse cross-section of the feed-wire is rotationallysymmetrical, however is reflectively non-symmetrical. That is, if thewire 11 is rotated through 180 degrees about its central longitudinalaxis, a first one of the major flanges 13 would occupy the same place,and have the same shape as a second one of the major flanges 13, andlikewise a first one of the minor flanges 14 would occupy the sameplace, and have the same shape as a second one of the minor flanges 14.However, no reflective plane of symmetry exists in transversecross-section. This is because the each of the major and minor flanges13, 14, while extending outwards from the core 12 does not extendexactly along a radius but rather is offset from the radius, and extendsparallel to the radius. Furthermore, each major flange 13 isnon-symmetrical about its centre line, and each minor flange 14 isnon-symmetrical about its centre line.

The depth 17 of each major flange 13 is greater than the depth 17 ofeach minor flange 14. The circumferential width of each minor flange 14is greater than the circumferential width of each major flange 13. Thusthe major flanges 13 are relatively slender since they have a greaterdepth 17 and narrower width and the minor flanges 14 are relativelysquat since they have a greater width and lesser depth 17.

When the feed-wire 11 of FIG. 3D is used with a plastic die, thetransverse cross-sectional shape of each grooves 3 is matched to theshape of the major flanges 13 when manufacturing the die 1. The majorflanges 13 will then be suited to fitting within the grooves 3 in thedie, whilst the core 12 and the minor flanges 14 are well suited tofitting within the cylindrical axial cavity 2, which is represented bythe dashed circular line in FIG. 3D.

The flanged feed-wires 11 of FIGS. 3A to 3D may be formed by taking acoil of stock wire having a circular transverse cross-section andshaping the stock wire, as it is wound from one reel to another. Theprocess involves progressively squeezing the stock wire between shapingrollers on a plurality of occasions such that the substantially circularcross-sectional shape of the stock wire is gradually plasticallydeformed into the desired cross-sectional profile without cutting orshearing the surface of the wire. The process produces a coil ofprofiled feed-wire having a plurality of longitudinal flanges extendingoutwards from its central core and running parallel to it.

In the process of deforming the stock-wire into feed-wire 11, thecross-sectional area of the wire reduces whilst the circumscribeddiameter of the wire increases. For example, for some wires thecross-sectional area reduces by at least 12.5%, to 40 square millimetresor less, whilst the circumscribed diameter of the wire increases by avalue in the range 45% to 85%.

The forming process work hardens the surfaces of the major flanges suchthat they are harder than the plastic die material. The tensile strengthof the wire, which is typically less than 600 Mps in its circular stockform, increases typically by a factor of around 1.7 to 2.3 in itsprofiled form, thereby delivering profiled feed wire having an ultimatetensile strength in the range 850 MPa to 1380 MPa.

It will be appreciated that the rolling process work-hardens theelongate flanges 13 such that they are harder than the central core 12.As a result, the softer core 12 of the feed-wire 11 remains sufficientlymalleable to be twisted whilst the hardened flanges 13 will not fold,crack or break during the twisting process.

The invention is not limited to twisting feed-wire 11 having the examplecross-sections shown in FIGS. 3A to 3D, or any combination of featurestaken therefrom. The examples, shown are merely for illustrativepurpose. The method and means of the current invention would be suitablefor any profiled metallic feed-wire having a plurality of roll-profiledlongitudinal flanges including a rhombus and including those where, intransverse cross-section, one flange is selectively adapted to bedifferent in orientation, depth 17, width and/or shape to at least oneother flange. It is possible that the die 1 can also be used withfeed-wires 11 that are produced by a different forming process.

Twisting Profiled Feed-Wire Using a Plastic Die

FIG. 4 is a schematic diagram illustrating the method of the currentinvention in which a profiled I feed-wire 11, which is typically madefrom steel, is forced in an axial direction through the plastic die 1and is extruded from the die 1 in the form of a helical shaped wire 15.

In high-pressure applications, steel sliding on a self-lubricatingsynthetic polymer plastic, such as nylon, has a lower coefficient offriction than that of steel sliding on steel. As mentioned previously,the plastic material used to form the die 1 can have a coefficient ofstatic friction on steel of less than approximately 0.3μ and, undernormal load, a coefficient of kinetic friction for steel on plastic ofless than approximately 0.2μ.

The cavity 2 within the plastic die 1 has an inscribed diameter that is25% to 50% less than the circumscribed diameter of the profiledfeed-wire 11. The arrangement is such that only the major flanges 13 arereceived in the internal grooves 3, and a clearance is provided betweenthe minor flanges 14 and cavity 2 wall.

Optionally, prior to inserting the feed-wire 11 into the mainmanufacturing die 1, it can be beneficial to pre-treat the leading endof the feed-wire 11. For example, in some instances it can be desirableto heat the leading end of feed-wire 11 to assist with the twistingprocess. It can also be helpful to pre-twist a leading end of thefeed-wire 11 to create an at least partially formed section of helicalwire 15 at the leading end. This makes it easier to insert the feed-wire11 into the die 1, and reduces wear of the die at the initialengagement. Pre-twisting a leading end of the feed-wire 11 can beperformed manually, and may include heating the leading end of the wire.In some arrangements, a second die (not shown) can be used to pre-twistthe wire. The second die is separate from the main twisting die 1. Thesecond die can have an axial cavity and helical grooves formed therein,in a similar arrangement to the axial cavity 2 and helical grooves 3formed in the main twisting die 1, but the second die is used only forthe purposes of preparing the leading end of the wire for insertion intothe main twisting die 1.

When the feed-wire 11 is ready, the leading end of the feed-wire isinserted into a leading end of the die 1, such that the core 12 andminor flanges 14 are inserted into the cavity 2, and the major flanges13 are inserted into respective grooves 3. For example, the die 1, canbe wound on to and pre-engaged with the leading end of the feed-wire 11,and is then fixed in place by a die holder 21 to lock the die 1 againstrotation during the twisting process.

The feed-wire 11 is driven in an axial direction through the die 1,whereupon the feed-wire 11 is twisted about its longitudinal axis tohelically shape the feed-wire 11. An active surface of an outer radialportion of each major flange 13 is deflected by an active surface of arespective helical groove 3, thereby creating turning moments acting onthe major flanges 13 which cause the axial core 12 to plastically deformin an helical manner. A helically-shaped wire 15 is extruded from adischarge end of the die 1.

The outer radial portion of each major flange 13 is, in transversecross-section, a portion measuring approximately 30% to 70% of theradial depth of the major flange 13 as measured from its tip towards theinscribed circumference of the core 12.

The extruded helically-shaped wire 15 is smaller in circumscribeddiameter than the feed-wire 11 from which it was produced. This isbecause the major flanges 13 on the feed-wire 11 are stressed intotension as they are stretched to extend both around and along thelongitudinal core 12 of the wire. This flange-elongation results in theextruded helical wire 15 having a circumscribed diameter that is up toapproximately 2.5% less than the circumscribed diameter of the feed-wire11.

The tensile forces imparted to the stretched major flanges 13 result ina reactive axial compression force being applied along the longitudinalcore 12 of the wire. This compression reduces the overall length of thetwisted wire 15 such that the wire extruded from the die 1 has a greatermass per unit length than the feed-wire 11. The helical wire 15 isapproximately 2% to 5% heavier than the feed-wire 11 of the same length.

In order to accommodate the physical changes that occur to the feed-wire11 during the twisting process the inscribed diameter of the cavity 2within the die is typically at least 30% greater than the inscribeddiameter of the central core 12 of the profiled feed-wire 11.

FIG. 5 is a schematic diagram showing a manufacturing system accordingto the invention. The manufacturing system includes profiled feed-wire11, a reel 22, a feed mechanism 23 a, the die 1, and a cutter 24. Thefeed-wire 11 is in the form of a continuous coil of wire and is storedon the reel 22. The reel 22 typically contains 100 kg to 800 kg offeed-wire 11, which depending on the gauge and composition of the wire11, represents around 500 m to 20,000 m of profiled feed-wire 11.

In its storage state, the feed-wire 11 is not straight; it is curved intwo dimensions by virtue of it being wound into a coil and stored on thereel 22.

During a set-up, the feed-wire 11 is inserted into the leading end ofthe die 1 such that the major flanges 13 are engaged within the die'sinternal helical grooves 3. The die 1 is wound on to the leading end ofthe feed-wire 11, thereby twisting the leading portion of the feed-wire11. The die 1 is then rotationally locked in a stationary position inthe die holder 21.

The feed mechanism 23 includes a plurality of drive rollers 23. Thedrive rollers 23 are driven by a suitable drive means, for example by anelectric motor, optionally via a transmission system. The drive rollers23 are arranged to pull the feed-wire 11 from the reel 22 and drivethrough the die 1. The curved form of the feed-wire 11 beneficiallyhelps the drive rollers 23 to grip the feed-wire 11 without slippage.The drive rollers 23 push the feed-wire 11 in an axial direction throughthe plastic die 1 without the need for sets of straightening rolls. Themajor flanges 13 act as radial lever arms which are deflected within theinternal helical grooves 3 by a plastic surface of the die as theyadvance through die 1. The die 1 twists and straightens the feed-wire 11as it passes through the die 1, and the wire is extruded from adischarge end of the die in the form of a helical wire 15 having aplurality of longitudinal helical threads, and typically a high-tensilehelical wire 15.

The feed-wire 11 is twisted at least one full rotation within theconfines of the die 1. This helps to substantially straighten the curvedfeed-wire 11 and to mitigate any helical spring-back recovery which mayotherwise result in the loosening of the helix as it is discharged fromthe die 1. Typically, the die 1 is arranged to twist the feed-wire 11through an angle in the range one to ten full rotations within theconfines of the die 1. In some embodiments, the die 1 is arranged totwist the feed-wire 11 through an angle in the range one and a quarterto twelve and a half rotations within the confines of the die 1. Thelow-friction plastic die 1 permits the feed-wire 11 to be drivensmoothly through the twisting die 1 at a throughput rate of around 0.1to 0.5 metres per second, enabling mass production of high-tensilehelical shaped wire 15 at commercially viable rates.

The drive mechanism 23 is adapted to stop and restart the drive rollers23 intermittently to allow a cutting arrangement 24 to cut or crop thehelical wire 15 to the desired length as substantially the whole reel ofwire 11 is fed through the straightening and twisting die 1 with minimalwastage. Optionally, this can be done automatically, for example by anelectronic controller, which synchronises operation of the feedmechanism 23 and the cutter 24. The low-friction die 1 is well suited toovercoming the alternating cycles of static and kinetic friction assequential lengths of helically-shaped wire 15 are cut.

The helically-shaped steel wire 15 may be further cut and/or furtherprocessed to form threaded ties, fasteners and rods having an axial coreand a plurality of longitudinal helical threads.

Whilst the example manufacturing system in FIG. 5 shows the apparatusand the coil of feed-wire 11 being in one orientation, the apparatus, orany part of it, and/or the coil of feed-wire 11 may be arranged in anyorientation suitable for feeding the feed-wire 11.

Helically-Shaped Wire

FIG. 6 shows a helically-shaped wire 15 produced by the manufacturingsystem. In this example the wire has a twisted axial core 12 with aplurality of major helical threads 13H twisted helically around the core12. The resultant helically-shaped wire 15 can be used to form at leastone of: a threaded tie, threaded fastener and threaded rod. Theresultant helically-shaped wire 15 has a substantially uniform leadangle B along the length of the wire 15. The lead angle B is typicallyin the range 50 to 70 degrees, which produces a relatively slack helicalformation, say compared to a wood screw. This enables thehelically-shaped wire 15, which has a suitably pointed or sharpened tip,to be driven percussively into a substrate, and therefore can functionas a fastener in the form of a helical nail. The resultanthelically-shaped wire 15 can also be used to secure together parts ofbrick walls that are separated by cracks. The slack helical formationfacilitates a good bond with mortar and the parts of the walls together,which makes it an effective tie. The lead angle B is the angle formed bythe helix and the central axis of the helically-shaped wire 15, it isthe complement of the helix angle A. The angles are defined by the leadmeasurement X and the circumference of a notional circumcircle aroundthe outside of the helical threads 13H.

Whilst each major helical thread 13H has a substantially uniform leadangle B (and hence a substantially uniform helix angle A), and thereforehas a substantially uniform lead measurement, the axial pitch distancefrom the peak of one thread and the peak of the next thread may varyalong the length of the tie, fastener or rod in some embodiments. Inother embodiments, the accuracy of the pitch is approximately equal tothe accuracy of the lead.

The lead X of a helical formation is the axial advance of a helix duringone complete turn (360°), that is, for a member having first and secondhelical threads intertwined with one another, the distance from a firstpeak of the first thread to the next peak on the first thread. The pitchY is the distance from one peak to next peak, that is for a memberhaving first and second helical threads intertwined with one another,the distance from a first peak of the first thread to the next peak,which is on the second thread

Thus when a helical member has a plurality of helical starts, thedefinition of the pitch measurement differs from the definition of thelead measurement.

In some embodiments of the invention, for helically-shaped wires 15having a plurality of helical threads 13H, the variation of the leadalong the length of each thread can be less than the variation of thepitch. The variation of the lead is calculated by subtracting theminimum lead measurement from the maximum lead measurement along thelength of the helical thread 13H measured. The variation of the pitch iscalculated by subtracting the minimum pitch measurement from the maximumpitch measurement along the length of the tie, fastener or rod.

For example, take a tie, fastener or rod having first and second helicalthreads 13H and a circumscribed diameter of say 10 mm, wherein each ofthe first and second helical threads 13H has a substantially uniformlead angle (and hence substantially uniform helix angle) and has a meanvalue of the lead measurements of 50 mm. The multiple lead measurementsof each given thread 13H may vary from the mean by plus or minus 0.25 mm(0.5% of the mean). In this example of a 2-start helix, the mean peak topeak pitch measurements would be 25 mm, though due to subtle deviationsin the cross-sectional depth 17, width, shape or orientation of eachmajor flange 13 in the feed-wire 11, the variation in the distance froma peak of the first thread 13H to an adjacent peak of the second thread13H may vary from the mean by up to approximately 4% of the mean. So, inthis example, the distance from a peak of the first thread 13H to a peakof the second thread 13H (or vice versa) may vary or alternate from 24.0mm to 26.0 mm.

Accordingly, for a helical member, for example in the form of a twistedtie, twisted fastener or twisted rod that has a plurality of helicalthreads 13H, the variation in the lead of least one of the helicalthreads 13H may be less than half the variation in pitch along thelengths of the threads. Variations in pitch may be useful in someapplications. For example, it can provide an improved mechanicalinterlock, for example when mortar is applied around the helical member.It may also be useful when nailing or tying dissimilar materials, suchas to connect a hard brick wall to a wall made of more friable materialsuch as aerated concrete of mortar-filled perforated masonry. A longerpitch distance provides a greater amount of material between theinterlocking peaks of the helix to enhance the shear strength at theconnection with the more friable masonry material.

The example shown in FIG. 6 is a helical member 15 having work-hardenedmajor threads 13H capable of cutting into a masonry-like substrate suchas brick, stone or concrete when percussively driven. The hardness of atleast one major thread is in the range 25 to 45 HRc. The helical member15 has a drivable tip formed at one end. A portion of the major threads13H has been removed at the other end of the member leaving a shankportion 16 having substantially parallel sides along at least a portionof the shank portion 16. The shank portion 16 has a maximum diameterthat is more than 50% of the maximum diameter of the helical member 15.The length of the shank portion 16 is less than or equal to the lengthof a pitch distance Y. In this example the shank portion 16 has atapered end.

Making the Plastic Twisting Die

FIG. 7 shows a typical tap 31 being used to cut helical grooves inside ablock of plastic, for example in a section of plastic rod, to form theplastic twisting die 1, which is suitable for twisting profiled steelfeed-wire 11. The block of plastic has a central longitudinal cavity 2bored through the material, for example by a drill.

The tap 31 has a plurality of cutting edges in the form of a pluralityof longitudinal helical threads 32. Each helical thread 32 has asubstantially uniform lead angle, and therefore substantially uniformhelix angle. The lead angle is typically in the range 50 to 70 degrees.

The tap 31 is aligned to the cavity 2 and is forced in an axialdirection into the plastic rod, for example by way of a percussive orpressing action. The plastic rod is restrained against rotation. Thehelical threads 32, which are tapered and are harder than the plastic,cut into the plastic. The lead angle of each thread 32 causes the tap 31to rotate as it advances through the cavity without any separaterotational force being applied.

As the tap 31 advances through the plastic rod, the helical threads 32form a plurality of internal helical grooves 3 within the wall of thecavity 2 along substantially the whole length of the cavity 2. Eachgroove 3 cut into the plastic material has a substantially uniform helixangle.

The circumscribed diameter of the helical threads 32 is greater than theinscribed diameter of the die's cavity 2. For the purpose of aligning orcentring the tap to the cavity 2, the tap 31 is shaped at one end 33.For example, one end 33 of the tap may be tapered and/or profiled toform a reduced diameter protrusion. The hollow plastic rod may have acountersink formed at one end of the cavity 2.

The helical threads 32 on the tap may be formed by a machining process.Alternatively, the tap 31 may be formed using a portion ofhelically-shaped wire 15 that has been made using the process shown inFIG. 4 and FIG. 5. For example, a helically-shaped wire 15 which hasbeen extruded from the plastic twisting die 1 can itself be adapted sothat it can be used as a multi-start tap 31 for forming helical grooves3 in plastic material to form a new die 1.

Producing a tap 31 from helical-shaped wire 15 having work-hardenedhelical threads 13H is much quicker, cheaper and more efficient thanforming a tap 31 using a machining process.

Although the present invention has been described in connection withspecific preferred embodiments and examples, it should be understoodthat the invention as claimed should not be unduly limited to suchspecific embodiments. Furthermore, it will be apparent to the skilledperson that modifications can be made to the above embodiment that fallwithin the scope of the invention. For example, while the die isdescribed as being a single piece of plastic, the die may instead beformed by several pieces of plastic fused end to end such that it isfunctionally a single article made of plastic prior to the formationinternal helical grooves.

The feed-wire 11 can have a different cross-section from the feed-wiresshown in FIGS. 3A-3D. The die 1 is arranged according to thecross-section of the feed-wire.

The feed-wire can be made from metals other than steel.

When manufacturing the die, instead of forcing the tap into arotationally restrained rod, the rod can be forced on to a rotationallyrestrained tap to produce the die body.

1. A method for manufacturing a helically-shaped member, the methodincluding: providing profiled feed-wire comprising metal, the feed-wirehaving a central core and a plurality of longitudinal flanges extendingoutwards from the core and running parallel to the core; providing a diebody made of plastic, the die body having an axial cavity formed throughthe die body and a plurality of internal helical grooves formed in thedie body, each helical groove extending substantially the whole lengthof the axial cavity at a substantially uniform helix angle; engaging atleast some of the flanges within respective internal helical grooves,and forcing the profiled feed-wire in an axial direction through the diebody, whereby a plastic surface within at least one of the helicalgrooves deflects its respective flange thereby forcing the profiledfeed-wire to helically deform as it advances through the die body; andextruding a helically-shaped wire from the die body.
 2. The methodaccording to claim 1, wherein the plastic comprises polyamide.
 3. Themethod according to claim 2, wherein the plastic comprises nylon.
 4. Themethod according to claim 1, wherein each flange extends radiallyoutwards from the core.
 5. The method according to claim 1, wherein theprofiled feed-wire includes steel.
 6. The method according to claim 5,wherein the profiled feed wire includes stainless steel.
 7. The methodaccording to claim 6, wherein the profiled feed-wire includesstainless-steel having an austenitic crystalline structure.
 8. Themethod according to claim 1, wherein the axial cavity has a uniformtransverse cross-section.
 9. The method according to claim 1, whereinthe axial cavity is substantially cylindrical.
 10. The method accordingto claim 1, wherein the grooves extend outwards from the axial cavity.11. The method according to claim 1, including helically deforming aleading end of the profiled feed-wire before inserting it into thetwisting die.
 12. The method according to claim 1, wherein an inscribeddiameter of the axial cavity within the die body is at least 30% greaterthan an inscribed diameter of the central core of the profiledfeed-wire.
 13. The method according to claim 1, wherein an outerdiameter of the die body is at least three times a circumscribeddiameter of the profiled feed-wire.
 14. The method according to claim 1,wherein the plurality of longitudinal flanges includes a plurality ofmajor longitudinal flanges and a plurality of minor longitudinalflanges.
 15. The method according to claim 1, wherein the die body hasfewer helical grooves than the feed-wire has flanges.
 16. The methodaccording to claim 1, wherein the profiled feed-wire is curved in form.17. The method according to claim 1, including winding the die body onto the leading end of a coil of feed-wire and locking the pre-engageddie body against rotation with a die holder.
 18. The method according toclaim 1, including driving the profiled feed-wire through the die bodyby a feed mechanism, the feed mechanism including powered drive rollers.19. The method according to claim 18, including temporarily haltingrotation of the powered drive rollers during an extrusion process, andsubsequently restarting the powered drive rollers.
 20. The methodaccording to claim 1, wherein the profiled feed-wire is twisted throughat least one full rotation within the confines of the die body.
 21. Themethod according to claim 1, wherein the profiled feed-wire is twistedthrough approximately 1.1 to 10 full rotations within the confines ofthe die body.
 22. The method according to claim 1, wherein thecoefficient of friction for steel sliding on the plastic used to makethe die body is less than the coefficient of friction of steel slidingon steel.
 23. The method according to claim 1, wherein the plastic has astatic coefficient of friction on steel of less than or equal toapproximately 0.3μ.
 24. The method according to claim 1, wherein theplastic has a kinetic coefficient of friction on steel of less than orequal to approximately 0.2μ.
 25. The method according to claim 1,wherein the modulus of elasticity of the feed-wire is greater than themodulus of elasticity of the die body.
 26. The method according to claim1, wherein the profiled feed-wire is driven through the twisting diebody at a throughput rate in the range 100 mm to 500 mm per second. 27.The method according to claim 1, wherein the helically-shaped wireextruded from the die body has a greater mass per unit length than theprofiled feed-wire that is fed into the die body.
 28. The methodaccording to claim 1, including cutting the helically-shaped wire toform threaded ties or rods.
 29. The method according to claim 28 whendependent on claim 19, including cutting the helically-shaped wire afterthe powered drive rollers have been halted.
 30. The method according toclaim 29, including a controller synchronising operation of a cuttingdevice with the powered drive rollers.
 31. The method according to claim1, including a preparatory step of providing stock-wire having asubstantially circular transverse cross-section and progressivelysqueezing the stock-wire between shaping rollers on a plurality ofoccasions such that the transverse cross-sectional shape of thestock-wire is gradually plastically deformed into a transversecross-sectional of the profiled feed-wire, without cutting or shearingthe surface of the wire.
 32. The method according to claim 31, whereinthe transverse cross-sectional area of the profiled feed-wire is atleast 10% less than the cross-sectional area of the stock-wire.
 33. Themethod according to claim 31, wherein a tensile strength of the profiledfeed-wire is in the range 1.7 to 2.3 times a tensile strength of thestock-wire.
 34. The method according to claim 1, wherein the profiledfeed-wire has an ultimate tensile strength in the range 850 MPa to 1380MPa.
 35. The method according to claim 1, wherein at least one of theflanges is work-hardened; the work-hardened flange extending outwardsfrom a softer core.
 36. The method according to claim 1, wherein atleast one flange is elongate in transverse cross-section.
 37. The methodaccording to claim 1, wherein at least one flange has a greater hardnessthan the die body.
 38. The method according to claim 1, wherein at leastone of the flanges tapers along its depth from a wider portion towards aroot portion to a narrower width at a tip portion.
 39. The methodaccording to claim 1, wherein the transverse cross-section of theflanges varies in depth, width or shape.
 40. The method according toclaim 1, wherein the die body is formed from a single piece of material.41. The method according to claim 1, wherein including providing acontinuous coil of profiled feed-wire wound onto a reel. 42.-89.(canceled)